Spinal midline indicator

ABSTRACT

A spinal midline indicator ( 10 ) has a body ( 14 ) of radiolucent material for insertion between adjacent vertebrae ( 18, 20 ) and a radiographic marker ( 12 ) located centrally with the body to indicate the position of the spinal midline ( 22 ) in anterior-posterior images when the body is centrally located between the vertebrae. The radiographic marker is typically an elongate metal handle. The body may carry secondary radiographic markers ( 16 ) on opposite sides of and equidistant from the handle so that the handle indicates the position of the spinal midline when the body is placed centrally between the vertebrae.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/616,697 (Attorney Docket No. 29850-705.302), filed Nov. 11, 2009,which is a continuation of U.S. patent application Ser. No. 11/187,733(Attorney Docket No. 29850-705.301), filed Jul. 21, 2005, which is acontinuation of International Application PCT/IB2004/000170 (AttorneyDocket No. 29850-705.601), filed Jan. 26, 2004, which claims the benefitof South African Application No. 2003/0874 (Attorney Docket No.29850-705.841), filed Jan. 31, 2003, the full disclosures of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates to a spinal midline indicator.

It is important for a surgeon performing an ALIF (anterior lumbarinterbody fusion) or ACIF (anterior cervical interbody fusion) cage orspinal disc replacement procedure to be able accurately to establish thecentre- or midline of the spine. It is only once the surgeon hascorrectly established the position of the spinal midline that he is ableto place the cage or spinal disc accurately on that midline. ff-centreplacement will result in eccentric loading and possible early failure oraccelerated wear.

At present, surgeons attempt to establish the spinal midline by visualinspection of an A-P (anterior-posterior) image. However this is ofteninaccurate, and can lead to subsequent off-centre placement of the cageor disc with potential disadvantages as described above.

The present invention seeks to provide an instrument which willfacilitate accurate establishment of the spinal midline.

BRIEF SUMMARY OF THE INVENTION

According to the present invention there is provided a spinal midlineindicator comprising a body of radiolucent material for insertionbetween adjacent vertebrae and a radiographic marker associatedcentrally with the body to indicate, in an anterior-posteriorradiographic image, the position of the spinal midline when the body isappropriately located between the vertebrae. Conveniently theradiographic marker is an elongate handle which is connected to the bodyto facilitate placement of the body between the vertebrae and which ismade of a radiographic material, i.e., a material which is substantiallyopaque to radiographic (fluoroscopic) imaging.

In the preferred embodiment, the body carries, in addition to the handlewhich serves as a first radiographic marker, two or more secondaryradiographic markers on opposite sides of and equidistant from the firstmarker, whereby the first marker indicates the position of the spinalmidline when the body is placed centrally between the vertebrae and thesecondary markers are seen in the radiographic image to be equidistantfrom lateral edges of the vertebrae.

Further according to the invention there is provided a method ofidentifying a spinal midline which comprises the steps of inserting thebody of a spinal midline indicator as summarized above between adjacentspinal vertebrae, manipulating the body so that the radiographic markeris seen in a radiographic image to be on the spinal midline, and, usingthe position of the radiographic marker as a guide, applying a marking,eg. a pin, to a vertebra to indicate the midline.

Other features of the invention are set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of exampleonly, with reference to the accompanying drawings.

FIG. 1 shows a perspective view of a spinal midline indicator accordingto the invention;

FIG. 2 shows a side view of the indicator in the direction of the arrow2 in FIG. 1;

FIG. 3 shows a side view of the indicator in the direction of the arrow3 in FIG. 1;

FIG. 4 shows an end view of the indicator in the direction of the arrow4 in FIG. 1;

FIG. 5 diagrammatically illustrates the indicator in use; and

FIG. 6 shows a diagrammatic cross-section at the line 6-6 in FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

The spinal midline indicator 10 seen in FIGS. 1 to 4 includes anelongate handle 12 and a body 14 carried centrally at one end of thehandle. The handle is made of a radiographic material, i.e. one which isopaque to radiation in the radiowave part of the spectrum, includingX-radiation. The handle may, for instance, be made of stainless steel ortitanium. The handle 12 extends substantially through the body 14. Thebody 14 is made of a radiolucent material, i.e. one which is at least tosome degree transparent to the radiation. The body may, for instance, bemade of PEEK (polyetheretherketone) or UHMWPE (ultra-high molecularweight polyethylene).

Embedded in the body 14 are two elongate markers 16, also ofradiographic material such as stainless steel or titanium. The markers16 are aligned parallel to the handle 12 and are located on oppositesides of, and equidistant from the handle.

FIG. 5 diagrammatically illustrates, in an anterior view, adjacent upperand lower vertebrae 18 and 20 respectively. As explained above it isimportant, during an ALIF or ACIF cage or spinal disc replacementprocedure carried out anteriorly, for the surgeon to be able accuratelyto establish the spinal midline, indicated by the line 22, since it iscentrally on this line that the replacement disc or cage must be placed.The procedure is typically carried out, with the patient lying prone andflat on his back, through a frontal incision.

In order to establish the midline 22, the surgeon aligns the handle 12at a vertical orientation and uses it to insert the body 14 between thevertebrae 18 and 20. It will be understood that a separate instrument100 is used to hold the vertebrae apart for this insertion to takeplace. An attempt is made to orientate the body centrally with thehandle 12 vertical, thereby to ensure that the handle correctlyindicates the midline 22.

An X-ray photograph or radiographic image is taken in the verticalanterior-posterior direction. In this radiographic image the handle 12,markers 16 and vertebrae 18,20 will be visible. By ensuring that themarkers 16 are equidistantly laterally spaced from the osseous edges 23of the vertebrae, i.e. that the distance 25 is the same on both sides,the surgeon can ensure that the body 14 and handle 12 are centrallypositioned. It will be understood that during this procedure, the handle12 itself operates as a radiographic marker indicating a centralposition.

It will also be understood that if the handle 12 and markers 16 arealigned with the anterior-posterior direction in which the radiographicimage is taken, they will appear in the radiographic image merely asdots of small lateral dimension. However if the handle is not perfectlyaligned in the anterior-posterior, i.e. vertical direction, parallaxeffects will result in the handle and markers being seen as lines ratherthan dots.

This is illustrated in FIG. 6 in which the full lines show the handle 10at the correct anterior-posterior or vertical orientation and the brokenlines show it at orientations in which it is misaligned by an angle 9.It will be understood that in a radiographic image in theanterior-posterior direction indicated by the arrow 24, the handle 12and markers 16 will appear as dots at the full line orientation but asshort lines at the broken line orientations.

By consulting radiographic images and manipulating the indicator 10 asnecessary in response to the information derived therefrom, the surgeoncan ensure that the indicator is at the correct position andorientation. When the indicator is in the correct position and at thecorrect orientation, the handle 12 will lie in a vertical planecontaining the midline 22. The surgeon can now use the handle as apositive indicator of that midline. The position of the radiographicmarker can be used as a guide to apply a marking to a vertebra toindicate the midline. He can accurately mark the midline, for instanceby knocking a pin 26 into one of the vertebrae.

Once the midline has been marked on one or both of the vertebrae, theindicator 10 is no longer required and can be removed for later re-use.The marker(s) then serve to indicate the midline 22 to enablesubsequent, accurate positioning of the relevant prosthesis to takeplace.

1. A spinal midline indicator comprising: a body of radiolucent materialsized and shaped to fit into an intervertebral space between twoadjacent vertebrae; and a radiographic marker formed as an elongatedhandle attached to and extending from the body along a central axis ofthe body in an anterior-posterior direction, wherein the handle isembedded in the body.